As text and image information generation and transmission becomes increasingly available in digitized, electronic form, the need for producing hard copies of such information directly with low cost, high speed printers increases. An attractive approach for satisfying this need is in the provision of a high-density, compact, linear array of LEDs used in electronic/optical printers. Besides the obvious advantage of compactness and increased density of light sources, the LED array system promises the advantage of being more reliable and less expensive for such use.
Important to printing applications, is the stringent requirement placed on the uniformity of the intensity of light emission from the LED arrays used in the electronic/optical printer. In display applications for LED arrays, uniformity is not nearly as critical as in a printer because the eye is a logarithmic detector and is relatively insensitive to light intensity variations. However, in printing applications, which involve relative movement of a linear array of LEDs across the photosensitive material, an intensity variation of as little as a few tenths of one percent will manifest itself in rasterbanding effects.
There are many aspects of materials used in fabricating solid-state, semiconductor light emitters which affect the performance of LED arrays used in printers, and the way in which the devices are made. Most of these considerations are well documented in textbooks such as "Light-Emitting Diodes" by A. A. Bergh and P. J. Dean published by Clarendon Press, Oxford, 1976 and "Injection Electroluminescent Devices" by C. H. Gooch published by John Wiley & Sons, London, 1973. Because of the completeness of this earlier literature, a detailed discussion of the LED array fabrication process is believed unncessary. However, one characteristic which is of particular concern during fabrication of LED arrays for use in optical printing applications, has to do with the production of dark-line defects in the completed LED arrays as discussed hereinafter. Dark-line defects in LEDs are caused by the presence of linear crystal irregularities frequently found in light emitting semiconductor wafers supplied by vendors of such materials. To overcome the effects of linear crystal irregularities in the fabrication of LED arrays is a principal purpose of this invention.